1. Field of the Invention
The present invention relates to a recording apparatus adapted for use as an output printer for a copying machine, a facsimile apparatus, a word processor or a host computer, or for use as a video output printer, and more particularly to a recording head having electrothermal converting elements and recording functional devices on the same substrate and adapted for use in such recording apparatus.
2. Related Background Art
A conventional recording head is constructed by forming an array of electrothermal converting elements on a monocrystalline silicon substrate, then arranging functional devices such as a transistor array, for driving said electrothermal converting elements, outside said silicon substrate, and connecting the electrothermal converting elements and the transistor array with a flexible cable or by wire bonding.
In order to achieve simplification of structure, decrease defects in the manufacture, improvement in uniformity of device characteristics and improvement in reproducibility in the above-explained structure, there is recently known an ink jet recording apparatus in which, as proposed in the Japanese Laid-open Patent Sho 57-72867, the electrothermal converting elements and functional devices are formed on a same substrate.
FIG. 5 is a partial cross-sectional view of such recording head, wherein shown are a semiconductor substrate 901 consisting of monocrystalline silicon; an N-type semiconductor collector area 902; an N-type semiconductor ohmic contact area 903 with a high impurity concentration; a P-type semiconductor base area 904; and an N-type semiconductor emitter area 905 of a high impurity concentration, and said areas constitute a bipolar transistor 920. There are further provided a silicon oxide layer 906 serving as a heat sink and insulating layer; a heat-generating resistor layer 907; an aluminum (Al) electrode 908; and a silicon oxide protective layer 909, and these layers constitute a substrate member 930 of the recording head, including a heat-generating part 940. A cover plate 910 defines a liquid path 950 in cooperation with the substrate member 930.
Although the above-explained structure is well designed, there is still a room for further improvement for sufficiently meeting the requirements of energy saving, high level of integration, cost reduction and satisfactory reliability needed in recent recording apparatus.
In the first place, for achieving commercial success, a recording head of high performance has to be supplied with a low price. For this purpose, a recording head of low cost has to be realized by integrating the functional devices at a high density and thereby reducing the area of the chip constituting the substrate member of the recording head.
Consequently, it has been attempted to realize a higher level of integration, by employing a shallower emitter area in the transistor serving as the functional device than in the above-explained structure, thereby reducing the design margin.
In such a base member for a recording head, a shallower structure of the diffused emitter area 905 allows limiting the lateral expansion of diffusion, thereby achieving a higher level of integration without sacrificing the voltage resistance, and also reducing the diffused capacity between the emitter area 905 and the base area 904.
However, ink jet recording with a recording head employing a substrate member obtained by forming the electrothermal converting elements on a substrate with such shallower base area has often resulted in failures in ink discharge. Analysis of this phenomenon has revealed that aluminum employed in the emitter electrode wiring 908 has caused a eutectic reaction with silicon contained in the substrate 901, thus developing alloy, called a spike, at the interface of the emitter area 905 and the emitter electrode, and said spike has reached the base area 904 penetrating the emitter area 905 and shortcircuiting the emitter and the base areas. In addition to such point requiring further improvement, following factors have to be taken into consideration.
On a substrate member for use in a recording head for the above-mentioned ink jet recording method, for example the one disclosed in the U.S Pat. No. 4,723,129 issued to Endo et al., there have to be formed electrothermal converting elements capable of generating thermal energy sufficient for inducing a state change in the ink and thereby discharging ink from discharge openings. On the other hand, functional semiconductor devices such as diodes or transistors have a temperature dependence in their characteristics and should therefore be operated, as far as possible, under stable temperature conditions.
Consequently, a completely new concept is required in the structure of the recording head and the substrate member therefor, in order to incorporate components of mutually controdictory properties on a same substrate member (including the case of forming the functional devices on a semiconductor substrate) and to achieve satisfactory functions of these components while preventing the formation of the aforementioned spikes. Besides, such recording head has to be formed with a low cost.